Gadhafi's Weapons Of Concern: Shoulder-Fired Missiles : The Two-WayThe "nightmare scenario," says policy analyst Frederic Wehrey, is that some of an estimated 20,000 such missiles fall into the wrong hands.

As opposition forces have moved into Tripoli, they've gotten into some of the Iraqi Libyan military's weapons depots. But they haven't necessarily secured all the military hardware, leaving some of it vulnerable to those who might try to sell it on the black market.

Of the weapons that Col. Moammar Gadhafi's forces have in their arsenals, those that officials from the U.S. and other nations most worry about getting into the hands of terrorists are an estimated 20,000 shoulder-fired surface-to-air missiles, says Frederic Wehrey, a senior policy analyst at the Rand Corp., U.S. Air Force reserve officer and — earlier this year — an attache at the U.S. embassy in Libya.

Wehrey told NPR's Melissa Block today that because the missiles "are very portable and hard to detect," they are potentially an "ideal terrorist weapon." They also require minimal training to use. Civilian airliners would be particularly vulnerable, he says.

Having those weapons in terrorists' hands, says Wehrey, "is a nightmare scenario."

It's his understanding, Wehrey added, that the U.S. and its allies are working with Libya's neighbors to try and prevent any smuggling of such arms. It's also possible, he says, that NATO nations might create a buy-back program to purchase the arms.

One other note: Melissa asked if Gadhafi's forces might have chemical or biological weapons that could get into the wrong hands if they aren't secured. "There's some ambiguity about that," says Wehrey. "He had a program, but I haven't seen firm evidence that these are real stockpiles or that they exist in any large quantity."

Melissa Block speaking with Frederic Wehrey

More from Wehrey's conversation with Melissa is due on today's All Things Considered. Click here to find an NPR station that broadcasts or streams the show. Later, we'll add the as-broadcast version of the interview to the top of this post.